Sputtering processes are used to apply a thin coating of a target material onto a substrate. Known sputtering processes typically take place in a partially evacuated chamber containing a small amount of ionizable gas such as argon. The gas is ionized by electromagnetic activation to form a plasma. A target material introduced into the chamber is bombarded by ions of the plasma, which imparts sufficient energy to eject atoms or molecules of the target material, which are then deposited onto the parts to be coated, i.e., the substrate.
Because the ejected atoms or molecules from the target material travel in a straight line, it is necessary to make special provisions for the coating of three dimensional objects having surfaces facing in more than one direction. It is known, for example, to move the parts to be coated while holding the target material stationary, thereby varying the "line of sight" from the target material to the substrate. U.S. Pat. No. 5,470,388 to Goedicke et al. discloses an apparatus for chemical vapor deposition or sputtering in which the parts to be coated are rotated in a drum at sufficient speed to press the parts against the wall of the drum by centrifugal force. A stripping device causes the parts to separate from the wall, fall downward by gravity, and rejoin the wall in a different position. The target material is held stationary within the rotating drum.
In U.S. Pat. No. 3,395,674 to Burnham et al., irregularly shaped parts are tumbled in a drum for coating in a vapor deposition process. The coating material is held stationary in an evaporator near the center of rotation of the drum.
The tumbling of parts has also been used in a chemical vapor deposition process wherein the coating material is introduced into the chamber in gaseous form. For example, a method and apparatus for making coated elastomeric closures by polymerizing a lower alkene with a plasma is disclosed in commonly assigned U.S. patent application Ser. No. 08/333,129 filed Nov. 1, 1994, which is hereby incorporated by reference in its entirety herein. That process uses a hollow tubular cylindrical reaction vessel for tumbling elastomeric closures in a low pressure environment. A gas is introduced containing a lower alkene such as propylene, as well as an inert gas such as helium or argon. Electromagnetic energy is applied to the vessel, causing a glow discharge in the interstices around the closures. The lower alkene is thereby deposited on the closures.
It is also known to mount the parts to be coated on a turntable or other means to present them at varying angles with respect to the target material. For example, in U.S. Pat. No. 4,952,295 to Kawabata et al., a turntable is placed in an evacuated chamber for rotation at relatively high speeds. The articles to be coated are arranged around the periphery of the turntable. Several pieces of target material are placed in positions near the turntable, and remain stationary with respect to the turntable. Several different target materials may be used in order to apply alternating layers on the articles.
One problem commonly encountered with such arrangements is that the target material coats not only the parts but also the walls and other elements within the chamber that are in a "line-of-sight" path with the target material. This secondary material transfer may consume a significant amount of target material, and necessitates a rigorous maintenance schedule in order to keep the functional parts within the evacuated chamber relatively free from the coating material.
As an alternative to moving the part to be coated with respect to the target material, it is also known to move the target material with respect to the part to be coated. A typical arrangement is disclosed in U.S. Pat. No. 5,527,439 to Sieck et al., wherein a cylindrical piece of target material is rotated with respect to an arrangement of permanent magnets that localize the plasma along a sputtering zone of the target. Material is ejected from the moving target surface onto a stationary substrate within the chamber. While this process may be used efficiently in the coating of flat objects such as silicon wafers, it does not lend itself to coating three-dimensional objects due to the "line-of-sight" problem discussed above.
Thus, there is a need for a bulk production coating apparatus that is capable of evenly and efficiently coating a quantity of three-dimensional objects. Despite considerable effort expended by those in the art, there has remained a significant, unmet need for an improved method for the bulk coating of three dimensional objects.